The difficulties encountered due to marine fouling are well documented in the literature. Slow moving and stationary surfaces that are exposed to marine environments become fouled because of the presence of various marine organisms including barnacles, Limnoria, various algal forms, etc. In this regard it is well known that such organisms attach themselves to concrete, wood and metal pier pilings, ship bottoms and the like and that removal of such organisms often requires expensive mechanical scraping or sandblasting. Certain organotin compounds, especially bis(tri-n-butyl)tin oxide, have been used as anti-fouling additives for coating compositions. However, coating compositions containing organotin oxides need to be re-applied periodically after immersion in a marine environment. For this reason, a continued search has been made for a compound or compounds which would have marine antifoulant activity at relatively low concentration levels while remaining effective as a marine antifoulant for long periods of time without being leached out by water or rain. It has now been unexpectedly discovered that the compound p-Dithiino(2,3-b) pyrazine-2,3-dicarbonitrile (hereinafter referrred to as Compound A) and the compound 1,3-Dithiolo(4,5-b)pyrazin-2-ylidene-propanedinitrile (hereinafter Compound B) have such a desirable combination of properties.